8431/16 RS422 C# programming (How to be sure all data is sent)

I am using the serial ports of 8431/16 card with C#.

I am using "Write" function of "SerialPort class" in .NET. "Write" function is a blocking function so I assume that when "Write" function return all data is sent, at least placed in transmit buffer. But I saw that for 9600 baudrate, writing above 10000bytes data size all data are no sent(about 9872 bytes sent when write function returns) to the other side when write function returns.(20000 bytes for 19200 baudrate).

I changed the value of transmit buffer from "SerialPort class" and also changed  8431/16 card transmit buffer size (1-127) but I could not solve the problem.

I want to be sure that my all data is sent to the other side so I can say the other side that I sent to you this match data.